Description:The rapper Nas had no shortage of people to thank at the debut of a documentary about his groundbreaking first album, “Illmatic.” Especially notable on the list: a certain tough-guy actor. “I’d like to thank Robert De Niro,” Nas said on Wednesday at the Beacon Theater screening, “because he’s always playing me.” It wasn’t just a good punch line. Like the “Raging Bull” star, Nas has made himself a rough symbol of this city. He’s a signifier of a certain period in New York’s history as well as an (0:52)